The present invention relates generally to interface devices and methods used to provide power to sensors and to provide normally open and/or normally closed outputs from the sensors to drive external loads.
Conventional sensor interface units provide a means for supplying power to a control sensor and for translating the sensor""s control outputs to drive external loads. These conventional interface units are typically used with various types of on/off control sensors, proximity detectors, level sensors, and any other devices that take or provide measurements that can be used to control devices or translated into control outputs, such as normally open and/or normally closed control outputs. The interface units are typically configured to control various types of loads such as, for example, starter relays connected to motors, solenoid actuators and programmable logic controllers (PLCs).
One conventional sensor interface unit is a standard two-wire interface, as shown in FIGS. 1a and 1b, that operates in either a normally open or a normally closed configuration (one or the other, but not both). In such a conventional two-wire interface, power for the sensor is provided from the current through the load, which requires a minimum leakage current through the load when the control output is in an inactive or off state. These two-wire interfaces may be designed to operate on AC power, DC power, or configurable to use either AC or DC power. Prototypical interface units of this type are model Bi25-G47SR-FX3X2, available from Turck USA and model NJ2-12GM50-WS, available through Pepperl+Fuchs.
A conventional variation on the standard two-wire interface unit is a field programmable unit that provides the ability to set the active state, i.e., either normally open or normally closed, via a switching mechanism in the field. For example, this may be provided by including a user interface on the sensor or by using additional wiring such as two terminals that are shorted to configure as a normally closed interface and left open to configure as a normally open interface.
Another type of conventional sensor interface unit is a three-wire unit in which power to the sensor is provided separately from the control outputs from the sensor, wherein the power and one of the control functions share a common wire. This type of configuration is illustrated in FIGS. 1c through 1f. FIGS. 1c and 1d show a three-wire normally open (FIG. 1c) and normally closed (FIG. 1d) source interface connected to a load device 110. The source interface 112 is powered through terminals 103 and 105. Switched power (in a source configuration) is available to the load between terminals 103 and 104. For the three-wire normally open source interface 112 shown in FIG. 1c, the load is switched on when the source interface 112 is active and switched off when the source interface 112 is inactive. For the three-wire normally closed source interface 112 shown in FIG. 1d, the load is switched on when the source interface 112 is inactive and switched off when the source interface 112 is active. FIGS. 1e and 1f illustrate a prior art three wire normally open (FIG. 1e) and normally closed (FIG. 1f) sink interface 114. The sink interface 114 is powered through terminals 106 and 108. Switched power (in a sink configuration) is available to the load between terminals 106 and 107. For the three-wire normally open sink interface 114 shown in FIG. 1e, the load is switched on when the sink interface 114 is active and switched off when the sink interface 114 is inactive. For the three-wire normally closed sink interface 114 shown in FIG. 1f, the load is switched on when the sink interface 114 is inactive and switched off when the sink interface 114 is active.
Yet another conventional interface unit includes four wires that is capable of simultaneously providing both normally open and normally closed control outputs. Two of the four wires are used to provide supply power to the sensor, one of the wires provides the normally open control output, and the last wire provides the normally closed control output. The additional fourth wire can be a significant disadvantage in small sensors, where space (especially wiring space) is at a premium.
While these above-mentioned sensor interface units are acceptable in certain applications, a need still remains for a sensor interface capable of simultaneously providing a normally open and a normally closed control output using only three wires. A preferred sensor interface would include a maximum of three external terminals for connection to three wires or conductors including a power terminal and terminals for connecting the sensor interface to load devices, such as a normally open and a normally closed load device. The sensor interface would preferably be adapted to control the normally open and the normally closed load devices concurrently.
The current invention provides a means for field wiring of conventional on/off type control sensors using a three-wire sensor interface that (i) delivers power to the sensor; (ii) provides a normally open control output from the sensor; and, concurrently, (iii) provides a normally closed control output from the sensor.
In one embodiment, the three-wire sensor interface is capable of being coupled to a sensor subsystem for conducting signals to and from the sensor subsystem. Preferably, the sensor subsystem provides normally open and normally closed control signals and the sensor interface provides interfaces for communicating these signals to control normally open and/or normally closed load devices. The preferred interface unit includes a plurality of external terminals including a normally open external terminal that may be electrically connected to one or more normally open load devices to control the operation of the normally open load devices, and a normally closed external terminal that may be electrically connected to one or more normally closed load devices to control the operation of the normally closed load devices. The interface unit also preferably includes an external power terminal for connection to a source of electrical power for supplying power to the sensor interface and, in one embodiment, to the sensor subsystem. A plurality of interface terminals are preferably electrically coupled between the interface unit and the sensor subsystem and include one or more control terminals, e.g., a normally open control terminal and a normally closed control terminal. The interface unit also includes an interface circuit adapted to receive the normally open and normally closed control signals from the sensor subsystem through the normally open and normally closed control terminals and to couple these control signals to the appropriate normally open or normally closed external terminals for controlling the external load devices.
In another embodiment, the interface circuit includes a normally open control path that receives the normally open control signal from the sensor subsystem and couples the normally open control signal to the normally open external terminal, and a normally closed control path that receives the normally closed control signal from the sensor subsystem and couples this signal to the normally closed external terminal. The interface circuit also receives supply power through the external power terminal from an external power supply. In one embodiment in which both types of load devices (normally open and normally closed) are connected, the circuit couples the supply power to the interface power terminal through the normally open control path when the normally open control signal is inactive or, alternatively, couples the supply power to the interface power terminal through the normally closed control path when the normally closed control signal is inactive. When used in a configuration in which only one type of load device is present, the circuit couples the supply power to the interface power terminal through either the normally open or normally closed control path depending on which type of device is connected to the sensor interface.